Abstract: A nonlinear total focusing method (NTFM) based on diffuse field information is proposed to solve the problem of low lateral resolution caused by the blind zone in near-field defect detection with ultrasonic phased array. While testing the defect of zirconium alloy plate, the defect signal close to the ultrasonic phased array probe is drowned in the blind zone. The proposed method uses the spatial coherence between signals to suppress noise and weaken the blind zone. In the experiment of two zirconium alloy plates with through-hole defects, the linear phased array with the center frequencies of 0.5 MHz and 1 MHz probes are used to excite the Lamb wave for obtaining the direct acquisition signals and diffuse field signals. After cross-correlation processing of the diffuse field signals, the reconstruction signal is obtained, and then combined with the direct acquisition signal to obtain a mixed signals. The NTFM of mixed signals is performed to obtain visualization of defects in zirconium alloy sheets. The experimental results show that compared with the traditional total focusing imaging method, the NTFM imaging suppresses artifacts, improves the imaging quality, and reduces the missed detection rate. The proposed method provides a new idea for improving the lateral resolution of near-field defect imaging of thin sheet materials.